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H-Bridge Problem

J

James

Jan 1, 1970
0
I am having a problem controlling an H-Bridge Circuit that I put
together, it is a variant of Eugene Blanchard's design. Anyway the
schematic can be found here:

http://www.geocities.com/jj_legend_83/circuit.jpg

I two power supplies, one of the driving voltage, and one for the power
to the gates of the MOSFETs. I have a function generator as a logical
input to the circuitry. The voltages that are involved are on the
circuit diagram. I have the grounds of the two bottom MOSFETs attached
to the negative terminal of the driving voltage DC power supply, and
the rest of the grounds are attached to the negative terminal of the
control voltage power supply. The floating terminal of the function
generator is also attached to the negative terminal of the control
voltage power supply.

Anyway, the upper-left BJT and MOSFET are always "on", and I am having
trouble figuring out why. If anybody could give me some pointers here
that would be great. Thanks!

- James
 
L

linnix

Jan 1, 1970
0
James said:
I am having a problem controlling an H-Bridge Circuit that I put
together, it is a variant of Eugene Blanchard's design. Anyway the
schematic can be found here:

http://www.geocities.com/jj_legend_83/circuit.jpg

I two power supplies, one of the driving voltage, and one for the power
to the gates of the MOSFETs. I have a function generator as a logical
input to the circuitry. The voltages that are involved are on the
circuit diagram. I have the grounds of the two bottom MOSFETs attached
to the negative terminal of the driving voltage DC power supply, and
the rest of the grounds are attached to the negative terminal of the
control voltage power supply. The floating terminal of the function
generator is also attached to the negative terminal of the control
voltage power supply.

Anyway, the upper-left BJT and MOSFET are always "on", and I am having
trouble figuring out why. If anybody could give me some pointers here
that would be great. Thanks!

Just peek at the circuit for a second, without knowing much of the
rest.
My suggestion is to try using a different MOSFET for the high side.
Or better yet, try another proven design.
 
James said:
I am having a problem controlling an H-Bridge Circuit that I put
together, it is a variant of Eugene Blanchard's design. Anyway the
schematic can be found here:

http://www.geocities.com/jj_legend_83/circuit.jpg

I two power supplies, one of the driving voltage, and one for the power
to the gates of the MOSFETs. I have a function generator as a logical
input to the circuitry. The voltages that are involved are on the
circuit diagram. I have the grounds of the two bottom MOSFETs attached
to the negative terminal of the driving voltage DC power supply, and
the rest of the grounds are attached to the negative terminal of the
control voltage power supply. The floating terminal of the function
generator is also attached to the negative terminal of the control
voltage power supply.

Anyway, the upper-left BJT and MOSFET are always "on", and I am having
trouble figuring out why. If anybody could give me some pointers here
that would be great. Thanks!

- James

Not a great design, the mosfets would be slower to turn off than on.the
upper left mosfet allways gets a current feed from the 33k base
resistor connected to A.
 
W

Walter Harley

Jan 1, 1970
0
James said:
I am having a problem controlling an H-Bridge Circuit that I put
together, it is a variant of Eugene Blanchard's design. Anyway the
schematic can be found here:

http://www.geocities.com/jj_legend_83/circuit.jpg

I two power supplies, one of the driving voltage, and one for the power
to the gates of the MOSFETs. I have a function generator as a logical
input to the circuitry. The voltages that are involved are on the
circuit diagram. I have the grounds of the two bottom MOSFETs attached
to the negative terminal of the driving voltage DC power supply, and
the rest of the grounds are attached to the negative terminal of the
control voltage power supply. The floating terminal of the function
generator is also attached to the negative terminal of the control
voltage power supply.

Anyway, the upper-left BJT and MOSFET are always "on", and I am having
trouble figuring out why. If anybody could give me some pointers here
that would be great. Thanks!

I don't think there's any way that circuit can work. Something is missing,
e.g., it's been copied incorrectly.

If the signal in is between 0v and 1v, then you're right, the upper-left BJT
will always be turned on. Its base would have to get close to Vcp for it to
turn off.
 
W

Winfield Hill

Jan 1, 1970
0
Walter Harley wrote...
James wrote ...

I don't think there's any way that circuit can work. Something is
missing, e.g., it's been copied incorrectly.

Or badly designed. :) E.g., how can one expect a 1V square
wave to switch an NPN at ground *and* a PNP at up to +18V???
If the signal in is between 0v and 1v, then you're right, the upper-
left BJT will always be turned on. Its base would have to get close
to Vcp for it to turn off.

It also suffers from basic architectural problems. The "signal"
should first be squared up and prepared as a valid logic (on/off)
signal, with one circuit. Then it can be inverted, level-shifted,
buffered, etc., and prepared for driving the MOSFETs. But this
needs to be done without delays in the wrong places, to prevent
high rail-to-rail shoot-through currents through the FETs.

Next the MOSFET gates need high drive currents to turn them on
and off. James' IRF3205 has of 3247pF gate capacitance, forming
a disastrous 32us time constant with a 10k pulldown resistor.
This big MOSFET hungers for amps of gate drive.

The easiest way to solve all these tough problems is to use an
inexpensive H-bridge driver. I like to use the Intersil HIP4080A
and 4081A for high-frequency bridges (e.g. 1MHz). There are many
other good choices for use below say 200kHz.
 
Hi

I'm Eugene Blanchard and I first designed and posted the H-bridge
schematics in 1994 to the newsgroups. There are several versions with
errors floating around. Since then I've refined them until the latest
version is located at http://www.geocities.com/fet_h_bridge/ I've
pretty well left them alone for the last couple of years until I run
across someone having problems...

I would check the wiring, the only reason the upper left BJT would
always be on is if the base resistor was grounded. A low turns on the
upper BJT. There's some problems with this design as the motor will
provide a large inductance to the circuit and cause a slow transition
when switching. This puts the mosfets into the linear mode where they
shouldn't be. They'll heat up and fry themselves.

Go to the link mentioned earlier and it discusses a better design. Good
luck
 
W

Winfield Hill

Jan 1, 1970
0
[email protected] wrote...
I'm Eugene Blanchard and I first designed and posted the H-bridge
schematics in 1994 to the newsgroups. There are several versions with
errors floating around. Since then I've refined them until the latest
version is located at http://www.geocities.com/fet_h_bridge/ I've
pretty well left them alone for the last couple of years until I run
across someone having problems...

Eugene, your design needs work. I'd like you to respond to the
post I made in this thread (the most recent post before yours).
 
T

Terry Given

Jan 1, 1970
0
Hi

I'm Eugene Blanchard and I first designed and posted the H-bridge
schematics in 1994 to the newsgroups. There are several versions with
errors floating around. Since then I've refined them until the latest
version is located at http://www.geocities.com/fet_h_bridge/ I've
pretty well left them alone for the last couple of years until I run
across someone having problems...

I would check the wiring, the only reason the upper left BJT would
always be on is if the base resistor was grounded. A low turns on the
upper BJT. There's some problems with this design as the motor will
provide a large inductance to the circuit and cause a slow transition
when switching. This puts the mosfets into the linear mode where they
shouldn't be. They'll heat up and fry themselves.

Go to the link mentioned earlier and it discusses a better design. Good
luck

less bad might be a better description.

Rg_on = 100R
Rg_off = 10k = 100*Rg_on

cross-conduction galore.

Cheers
Terry
 
W

Winfield Hill

Jan 1, 1970
0
Terry Given wrote...
I guess not....

Perhaps my comment wasn't strong enough. The design should be
immediately removed from circulation, before it hurts anything.
 
T

Terry Given

Jan 1, 1970
0
Winfield said:
Terry Given wrote...



Perhaps my comment wasn't strong enough. The design should be
immediately removed from circulation, before it hurts anything.

And worse still, teaches a whole new generation of electronics
enthusiasts to design circuits that dont work.

Cheers
Terry
 
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